1. Field of the Invention
The present invention relates to an original reader for photoelectrically reading an original image and forming an image signal.
2. Description of the Prior Art
A known line sensor has an array consisting of a plurality of photosensor elements of amorphous silicon or the like arranged widthwise (i.e., across the width) of an original for photoelectrically reading a density of an original image. When an A4 size original is read in the equal size copy mode at a resolution of 16 pixels/mm along the widthwise direction (about 210 mm), a line sensor consisting of about 3,500 photosensor elements on a substrate having a side of about 300 mm is required. However, it is expensive to form such a large number of photosensor elements on a single substrate without errors and with uniform sensitivity unless a great improvement in manufacturing yield is made.
It is possible to use a plurality of line sensors each having 1,000 photosensors arranged along the scanning direction to read an image in a divided manner. With this method, since the number of photosensors to be formed on a single substrate is reduced, the problem of cost due to a low manufacturing yield can be resolved to some extent.
However, non-effective bits which do not contribute to image reading are present at the ends of each line sensor. Therefore, when a plurality of line sensors are arranged on a line, gaps occur. To prevent this, the line sensors can be staggered so that the reading lines of the adjacent line sensors overlap each other.
When a plurality of line sensors are staggered, if adjacent line sensors read an original image by moving vertically relative to each other along different original image portions in the reading/scanning direction, there is a time lag between signals from the first array of line sensors and those from the second array.
Because of this time lag, in a copying machine or the like having a high resolution, of 16 pixels/mm, for example, the image formed is adversely affected. The time lag also adversely affects the color balance when an original image is a color image.
When an original image is reproduced on an enlarged or reduced scale, the relative speed between an original to be read and the line sensor is changed in accordance with a selected magnification. When image reading at a desired magnification is performed with a line sensor consisting of a plurality of line sensors arranged in a staggered manner, time lag between reading of an image line read by a previous line sensor and that by a next line sensor varies in accordance with a selected magnification. Therefore, the problem of time lag due to the specific arrangement of the line sensor is made worse.